The present invention relates generally to a method and apparatus for measuring aqueous erosion and transport from a sediment core sample. More specifically, the present invention relates to an adjustable shear stress erosion and transport flume having a variable-depth sediment core sample with the capability to simulate complex wave action with, or without, a superimposed unidirectional flow.
Many contaminants are sorbed to sedimentary particles and are buried at depths of up to several meters in the bottom sediments of rivers, lakes, estuaries, navigation channels, beaches and near-shore areas of the oceans. An important question is whether these buried sediments (and associated contaminants) can be exposed and transported during large floods and storms, releases from dams, etc., either with or without oscillatory wave action. In order to answer this question, knowledge of the erosion and transport properties of sediments at high shear stresses (up to and exceeding 10 Pa) and with depth through the sediment layer (up to and exceeding one meter) is needed.
To characterize the movement of sediments in aquatic systems one must not only have an understanding of the bulk erosion rates of sediments, but also be able to distinguish between two primary modes of sediment transport, i.e., suspended transport and bedload transport. As shown in FIG. 1, suspended transport of a sediment grain or particle in flowing water occurs when the vertical component of the turbulent flow velocity is approximately equal to or greater than the settling (i.e., falling) speed of the grain. Unsuspended transport, also known as bedload transport, includes a variety of transport mechanisms, such as saltation, rolling, sliding, and tumbling. Saltation occurs when a particle momentarily leaves the bed and rises no higher than a few grain diameters. Rolling, sliding, and tumbling are additional processes wherein particles are transported along the bed primarily by the horizontal force of the overlying flow of water. In bedload transport, the particles receive no significant upward impulses other than those due to successive contacts between the solid and the bed; the fluid impulses on the grains being essentially horizontal. Saltation transport is generally included in the category of bedload transport since saltation is restricted to only a few grain diameters in height above the bed.
Erosion and transport of sediments in rivers and streambeds, along ocean beaches, harbors, navigation channels and waterways, and around bridge support structures, is a complex process that depends on many variables. Sediments may erode particle-by-particle (e.g., sand and gravel), or may erode as aggregates or chunks, especially if the particles are fine-grained and cohesive (e.g., clay or silt). The aggregates/chunks can vary in size from microns to centimeters; generally do not re-suspend; and are made from very fine-grained particles that likely would re-suspend if disaggregated.
Sediments may also be contaminated with chemical, biological, or industrial contaminants, which can affect the degree of cohesiveness. Erosion and transport rates can also depend on grain size, shape, density, degree of cohesiveness, chemistry, organic content, and gas content. As shown in FIG. 2, aggregated (cohesive) particles eroded from the bed at an upstream position, X1, can de-aggregate at a downstream position, X2, due to subsequent impacts and collisions with the channel bottom and/or other aggregates and particles (e.g., during saltation transport).
Erosion rates and transport modes also depend on the shear stress applied across the sediment""s surface by velocity of the flowing liquid (e.g., water). Typically, a threshold exists where no appreciable erosion occurs below a critical shear stress. The critical shear stress for erosion may depend on whether or not the flow is unidirectional, oscillatory, non-uniform/irregular, or combinations of these.
Accurate prediction of erosion rates (bulk/total, suspended, and bedload) and subsequent transport and re-deposition for each mode of transport (suspended or bedload) is complicated by a lack of understanding of the cohesive forces that bind together fine-grained sediments (especially for contaminated sediments). Therefore, a need exists for an apparatus that can accurately measure the individual contributions to the total erosion rate of sediments from suspended and bedload erosion processes, whether in the laboratory or in the field, including oscillatory flow.
A previous apparatus for measuring bulk erosion of sediments, called a xe2x80x9cSEDflumexe2x80x9d, is described in xe2x80x9cMeasurements of Erosion of Undisturbed Bottom Sediments with Depthxe2x80x9d, J. McNeil, C. Taylor, and W. Lick, Journal of Hydraulic Engineering, June, 1996. A similar device is described in U.S. Pat. Ser. 6,260,409 to Briaud, et al., xe2x80x9cApparatus and Method for Prediction of Scour Related information in Soilsxe2x80x9d. However, these devices can only measure the total (i.e., bulk) erosion rate of a sediment core sample; they cannot independently measure the separate contributions from suspended and bedload erosion sources.
U.S. Pat. No. 6,494,084, xe2x80x9cAdjustable Shear Stress Erosion and Transport Flumexe2x80x9d, to Roberts and Jepsen, which is incorporated by reference, describes an improvement to the SEDflume device. Certain embodiments of this invention comprise a SEDflume type device with one or more traps (e.g., capture basins) located downstream of the sediment core sample. The bedload sediments (i.e., particles and aggregates) transported in the flow stream are gravitationally separated and captured in the downstream traps. The use of downstream traps allows measurement of the individual contributions of the total erosion rate from sediments suspended in the flow stream; and from bedload sediments transported along the floor (i.e., bed) of the channel.
However, none of the devices described above have means for simulating complex wave action (including oscillating flow) either alone, or in combination with, a superimposed unidirectional flow.
Hence, a need still exists for a device that can measure sediment transport properties of cohesive and non-cohesive sediments with depth and at high shear stresses due to complex wave action with, or without, a superimposed unidirectional current. Against this background, the present invention was developed.
The present invention relates to a method and apparatus for measuring erosion rates of sediments and at high shear stresses due to complex wave action with, or without, a superimposed unidirectional current. Water is forced in a channel past an exposed sediment core sample, which erodes sediments when a critical shear stress has been exceeded. The height of the core sample is adjusted during testing so that the sediment surface remains level with the bottom of the channel as the sediments erode. Complex wave action is simulated by driving tandom piston/cylinder mechanisms with computer-controlled stepper motors. Unidirectional flow, forced by a head difference between two open tanks attached to each end of the channel, may be superimposed on to the complex wave action. Sediment traps may be used to collect bedload sediments. The total erosion rate equals the change in height of the sediment core sample divided by a fixed period of time.
Sandia National Laboratories (SNL) has designed, constructed, and tested a high shear stress flume that can superimpose an complex wave action upon a unidirectional current. This apparatus is named the Sediment Erosion Actuated by Wave Oscillations and Linear Flow (SEAWOLF) flume. The SEAWOLF flume can be housed in a self-contained, mobile trailer and used on-site in research and mission support investigations of combined unidirectional current and wave induced erosion of in-situ contaminated sediments, dredged material mixtures composed of cohesive and non-cohesive sediments, or other sediments.
Results from hydrodynamic modeling of the SEAWOLF flume indicate oscillatory flow regimes in the SEAWOLF flume induce shear stresses up to 10 Pa. The addition of unidirectional flow can induce shear stresses greater than 12 Pa. Erosion experiments were performed using the SEAWOLF apparatus under a range of unidirectional and oscillatory flow combinations. These experiments verified model predictions that the undeveloped oscillatory flow shear stresses are much greater than those generated by fully developed, unidirectional flow. Effective shear stresses were determined from erosion tests with known sediment samples, making SEAWOLF a useful tool for predictive modeling.
The SEAWOLF apparatus may also be used to measure the critical shear stress necessary to initiate erosion.
The SEAWOLF flume may incorporate one or more upstream or downstream sediment traps. Eroded bedload sediments are transported downstream are gravitationally separated from the flow stream into one or more sediment trap (i.e., capture basins). After a known period of time, the bedload sediments (both particles and aggregates) that were captured in the trap(s) are weighed and compared to the total mass of sediment eroded (as measured by the change in height of the core sample), and may also be compared to the concentration of sediments suspended in the flow stream.
The SEAWOLF flume is described in Development of Flume with Oscillatory Flow Superimposed over a unidirectional Flow, Sandia National Laboratories, http://www.nwer.sandia.gov/wlp/factsheets/oscflow.pdf, Jul. 1, 2001, which is incorporated herein by reference.
The SEAWOLF flume is also described in The SEAWOLF Flume: Sediment Erosion Actuated by Wave Oscillations and Linear Flow, by Richard Jepsen, Jesse Roberts, Joseph Z. Gailani, and S., Jarrell Smith, Sandia National Laboratories technical report SAND2002-0100, January, 2002, which is incorporated herein by reference.